Slipping the Shumagin Gap: A Kinematic Coseismic and Early Afterslip Model of the Mw 7.8 Simeonof Island, Alaska, Earthquake. Issue 19 (1st October 2020)

• Record Type: Journal Article
• Authors: Crowell, Brendan W.; Melgar, Diego
• Is Part Of: Geophysical research letters. Volume 47:Issue 19(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-10-01
• Subjects: Shumagin Gap -- Alaska earthquakes -- earthquake rupture modeling -- high‐rate GNSS -- strong‐motion seismology; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2020GL090308
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: In July 2020, a Mw 7.8 earthquake initiated directly to the east of Simeonof Island offshore of the Alaska Peninsula. The earthquake ruptured the eastern part of the Shumagin Gap, a region devoid of large earthquakes over the last century and characterized by low geodetic coupling. Here, we investigate the rupture kinematics of the earthquake using a joint inversion of high‐rate GNSS and strong‐motion data. We find that the rupture was focused between depths of 30–45 km, starting east of the Shumagin Islands and rupturing downdip towards the northwest, with little slip west of 160°W. Early postseismic observations indicate that the entirety of the Shumagin Gap at depths between 40–60 km ruptured with aseismic afterslip and aftershocks. Historically, this earthquake resembles the Shumagin Islands earthquake of 1917, indicating that a possible rupture asperity exists to explain low interseismic coupling and repeating ~M7.5–8 earthquakes. Plain Language Summary: A large Mw 7.8 earthquake occurred in July 2020 in the Aleutian Islands near a part of the subduction zone that is not locked, the Shumagin Gap. A fully locked fault will be more susceptible to large earthquakes since deformation is not released slowly over time. We model how the earthquake slipped using both observations of displacement and velocity nearby. We find that the July 2020 earthquake ruptured mainly the unlocked portion of the subduction zone and did not rupture into regions that are highly locked. This peculiar pattern of slip was also seen previously in 1917, indicating that the structure of the fault zone in the area may be conducive to earthquakes and some interseismic locking is occurring to allow for M7.5–8 earthquakes every century. Key Points: The Simeonof Island earthquake ruptured a region of low interseismic coupling in the Shumagin Gap Early postseismic deformation indicates slip along the whole width of the Shumagin Gap between 40 and 60 km The Simeonof Island earthquake resembles the 1917 earthquake, indicating a potential rupture asperity within a highly creeping region